In the preparation of rubber compositions for manufacturing bead apexes of tires, importance has been attached to increasing their complex elastic modulus (E*) to enhance handling stability. Also in recent years, the growing demand for more fuel-efficient cars has led to the need not only for rubber compositions for manufacturing treads that account for a large portion of a tire but also for rubber compositions for manufacturing bead apexes to have better fuel efficiency (low heat build-up properties).
One way of increasing E* is, for example, to use a large amount of carbon black with a large reinforcing effect, but in this case, fuel efficiency tends to be deteriorated. Meanwhile, to improve fuel efficiency of rubber compositions, a method of reducing the amount of carbon black or a method of using carbon black with a large particle size may be employed. These methods, however, decrease the E* of the rubber compositions. This decrease in E* is not desirable because it can cause reduction in handling stability and lateral rigidity of tires. Another proposed method for improving fuel efficiency involves using silica. This method, however, is not desirable either because it tends to decrease extrusion and building processabilities of rubber compositions. Thus, there is a need for a method capable of simultaneously ensuring E* and fuel efficiency.
Moreover, rubbers for sidewall reinforcing layers (inserts) for run-flat tires are expected to be capable of supporting the load of a vehicle to allow the vehicle to run a certain distance with the tire having no internal pressure after a puncture is made in the tire. Accordingly, high-hardness properties, elongation at break, and low self-heating properties are important factors. Also, since these rubbers use a large amount of modified polymer, it is also important that they have excellent processability before vulcanization.
A possible method to provide both high hardness and fuel efficiency is to reduce the amount of carbon black and add a large amount of crosslinking agent such as sulfur or accelerators. This method, however, unfortunately greatly decreases elongation at break and thus greatly reduces tensile strength. The method is also associated with the problem of reduced run-flat durability. Another possible method involves using low-grade carbon black such as FEF and GPF. However, unfortunately, this method cannot simultaneously provide high hardness and fuel efficiency and even decreases elongation at break. This method further involves the problem of reduced run-flat durability.
Patent Literatures 1 to 3 suggest the use of a modified rubber such as modified butadiene rubber or modified styrene-butadiene rubber to reduce rolling resistance. Still, these rubber compositions have room for improvement in terms of improving high-hardness properties, elongation at break, and self-heating properties and eventually run-flat durability while maintaining good processability.